1. Field of the Invention
The invention relates to a field emission cold cathode emitting electrons from a thin electron-emission layer, and also to a display employing the field emission cold cathode to display visual information, such as a planar display device.
2. Description of the Related Art
There has been suggested field emission cold cathode array (FEA) including a plurality of micro cold cathodes arranged in an array, each micro cold cathode comprising a fine conical emitter and a gate electrode disposed in the vicinity of the emitter and having functions of generating a current through an emitter and controlling the thus generated current. For instance, such field emission cold cathode array has been suggested by C. A. Spindt et al. in "A Thin-Film Field-Emission Cathode", Journal of Applied Physics, Vol. 39, No. 7, June 1968, pp. 3504-3505, and by H. F. Gray.
The suggested FEA has advantages over a thermionic cathode that it can provide a higher current density, and that it has smaller dispersion in velocity of electrons emitted from an emitter. Furthermore, FEA makes smaller current noises than a single tip field emission cathode, and can operate even with a small voltage in the range of tens of volts to 200 volts even in an environment of relatively poor degree of vacuum.
FIG. 1 illustrates a conventional planar display apparatus suggested by R. Meyer et al. in "Recent Development on "Microtips" Display at LETI", Technical Digest IVMC 91, Nagahama 1991, pp. 6-9, where a plurality of FEAs 70 as electron sources are arranged in column and row. FEAs 70 emit electrons to a phosphor layer (not illustrated) disposed in facing relation with FEAs 70 to thereby cause the phosphor layer to emit lights. The illustrated planar display apparatus has advantages over a cathode ray tube (CRT) display apparatus that it is smaller in volume and weight, it consumes less power, and it can display images with higher accuracy. In addition, the illustrated planar display apparatus has advantages over a liquid crystal display (LCD) apparatus that it consumes less power, and it has wider field view angle because a phosphor layer in the illustrated planar display apparatus emits spontaneous lights.
There has been suggested a display device as electron sources where a diamond thin film having a small work function is employed, and it is unnecessary to fabricate a micro-structured device unlike the above-mentioned FEA. This display device is called an electron-emission electronic device. FIGS. 2A and 2B illustrate an example of electron-emission electronic device which has been suggested in Japanese Unexamined Patent Publication No. 6-36680.
FIG. 2A is a plan view and FIG. 2B is a side view of the suggested electron-emission electronic device. As illustrated, the electron-emission electronic device 100 includes a support substrate 103, a diamond electron-emitter 101 formed on the support substrate 103, and an anode 102 formed on the support substrate 103 in facing relation with the diamond electron-emitter 101. The diamond electron-emitter 101 is constituted of a thin monocrystalline diamond film or a thin polycrystalline diamond film, and is adhered onto the support substrate 103.
A diamond crystal has a work function smaller than that of metal and semiconductor such as silicon, and accordingly can emit electrons in an electric field having a quite small intensity. Specifically, metal and semiconductor have a critical electric field, at which electrons are emitted, of about 3.times.10.sup.7 V/cm. In contrast, a diamond has a critical electric field of about 5.times.10.sup.5 V/cm, which is two orders smaller than that of metal. Hence, the electron-emission electronic device is not required to have a quite sharpened structure for concentrating an electric field, and have a microstructure, unlike the above-mentioned FEA.
Japanese Unexamined Patent Publication No. 6-208835 has suggested a planar display apparatus employing a diamond layer as electron sources, which is illustrated in FIGS. 3A, 3B and 4. FIGS. 3A and 3B are cross-sectional views illustrating a single pixel in the planar display apparatus, and FIG. 4 is a perspective view illustrating the planar display apparatus employing the pixels illustrated in FIGS. 3A and 3B.
With reference to FIGS. 3A, 3B and 4, a plurality of stripe-shaped first conductive layers 112 are formed on a substrate 111, and the stripe-shaped first conductive layers 112 are covered with a phosphor layer or a cathode luminescence layer 113. A face plate 114 is spaced away from the substrate 111 in facing relation. A space between the substrate 111 and the face plate 114 is kept vacuous. A plurality of second conductive layers 115 extending perpendicularly to the first conductive layers 112 are formed on the face plate 114. A plurality of diamond layers 116 having the same width as that of the second conductive layer 115 are formed on the second conductive layers 115.
A section defined by intersection of the first conductive layer 112 with the second conductive layer 115 establishes a pixel. By applying a voltage across the first and second conductive layers 112 and 115, the diamond layers 116 emit electrons, which impinge on the phosphor layer 13 to thereby cause the phosphor layer 13 to emit lights.
As illustrated in FIG. 5, there has been suggested a display structure where a plurality of stripe-shaped grids 117 are supported by grid supports 118 between a cathode comprised of diamond layers 116 and a face plate 114, by N. Kumar et al. in "Development of Nano-Crystalline Diamond-Based Field-Emission Displays", SID 94 DIGEST, 1994, pp. 43-46.
Japanese Unexamined Patent Publication No. 7-272618 has suggested an electron source where an insulating film 124 and a gate electrode layer 125 are formed on a thin diamond film 123, as illustrated in FIG. 6.
Japanese Unexamined Patent Publications Nos. 8-77917 and 8-77918 have also suggested a field emission device including an electron source comprising a thin diamond film on which an insulating film and a gate electrode layer are formed.
As illustrated in FIG. 7, Japanese Unexamined Patent Publication No. 8-505259 corresponding to the international patent application PCT/US93/11791 or U.S. patent application Ser. No. 07/993,863 has suggested an electron source where thin, planar diamond films 131 are formed on bottom surfaces in cavities defined by a plurality of insulating films 124 and gate electrodes 125 formed on the insulating films 124.
As illustrated in FIG. 8, Japanese Unexamined Patent Publication No. 8-115654 has suggested an electron source a thin film 141 is formed on a bottom surface in a cavity defined by an insulating layer 142 and a gate electrode 144 formed on the insulating layer 142.
In a planar display apparatus including FEA where a plurality of fine sharpened emitters are arranged in an array, a plurality of micro-structures where a curvature radius of a tip end of emitters is equal to or smaller than 10 nm and a diameter of openings formed in a gate electrode are about 1 .mu.m have to be formed all over a display panel. To this end, it would be necessary to use the latest lithography apparatus. In particular, it would be necessary to use an exposure apparatus having high resolution in order to expose resist to light for forming gate openings.
However, it would be impossible to widen an area for forming a pattern therein in such a high-resolution exposure apparatus. Accordingly, it would be necessary to repeatedly move the exposure apparatus to cover a wide area for accomplishing a wide area display. As a result, it would be unavoidable that a time for operating the exposure apparatus is longer and longer, and hence it would take much time to complete an exposure step. In addition, it would be quite difficult to fabricate emitters in an entire display area so that the emitters have a tip end having a uniform curvature radius and also have a uniform height in an evaporation step for forming emitters in Spindt type or in an etching step in Gray type.
Since the planar display apparatus illustrated in FIGS. 3A, 3B and 4 has an electron source comprised of the diamond layers 116 having a small work function, it is no longer necessary to fabricate a micro-structured device by photolithography, and it is also unnecessary to use a high resolution exposure apparatus, which ensures that fabrication steps are simplified, and that the planar display apparatus could have a simpler structure.
As mentioned earlier, the planar display apparatus controls electron emission by a voltage to be applied across the cathode or second conductive layer 115 and the anode or first conductive layer 112 covered with the phosphor layer 113. Since the cathode 115 is spaced away from the anode 112 by a distance in the range of about 10 .mu.m to about 100 .mu.m, it would be necessary to apply a voltage in the range of 300 V to 500 V across the cathode 115 and the anode 112 for establishing an electric field sufficiently intensive for electron emission. Hence, even the fact that voltage-current characteristic is non-linear is utilized, the voltage has to be in the range of +80 V to +150 V for modulating a current. In general, a planar display apparatus is required to have driving circuits by the number equal to the number of horizontal and vertical pixels. Accordingly, if a current modulating voltage were great, external driving circuits would have to receive quite large burden.
In addition, when a voltage applied across the anode and the cathode is varied, an acceleration voltage for causing electrons to impinge on the phosphor layer would vary similarly to an emission current. Thus, it would be difficult to accurately adjust primary colors balance in a color display, for instance.
In addition, since the thin diamond films 116 do not always have a uniform micro-structure, a part of the emitted electrons have a horizontal velocity ingredient, and hence are not directed perpendicularly to the face plate 114 and the substrate 111. Accordingly, electrons to be emitted to a certain pixel may reach an adjacent pixel, which is accompanied with a problem that resolution and contrast is reduced in a display panel, in particular, color purity may be deteriorated in a planar color display apparatus.
For instance, when a voltage of 200 V is applied across the anode and the gate electrode and the anode is spaced away from the gate electrode by 50 .mu.m, an electron having been emitted by an angle of 30 degrees from a central axis would be radiated on a location remote from the central axis by about 15 .mu.m in a screen on which the anode is formed.
In order to solve the above-mentioned problem, it would be necessary to design the phosphor layer to have a larger area relative to an area of an anode in a pixel, design a spacing between the anode and the phosphor layer to be smaller to thereby ensure that electron beams certainly impinge on the phosphor layer before the electron beams diverge, or form a barrier wall for physically banning electrons to reach an adjacent pixel. However, these solutions to the above-mentioned problem would cause another problem that the planar display apparatus would have limited definition, and/or would have a more complex structure.
The display apparatus illustrated in FIG. 5 has a problem that the display apparatus cannot avoid to have a complex structure because the grid 117 having square apertures a side of which is in the range of 1 .mu.m to a few .mu.m has to be supported between the face plate 114 and the electron source. In addition, it would be difficult to fabricate the grid 117 having the micro apertures as mentioned above.
In the electron source illustrated in FIG. 6, since projections and recesses at a surface of the thin diamond film 123 are not always arranged in a line, emitted electrons tend to have a large horizontal velocity ingredient. In addition, in a cavity defined by the insulating layers 124 and the gate electrode layers 125, there does not exist a focusing electric field directing towards a center of the gate opening, and hence a majority of emitted electrons impinge on the gate electrode layers 125. A few electrons can pass through the openings of the gate electrode layer 125. As a result, the gate electrode layer 125 is heated, and power consumption due to that cannot be disregarded. Furthermore, a temperature around the gate electrode layer 125 is caused to be increased, resulting in deterioration in a degree of vacuum inside the electron source.
The electron source illustrated in FIG. 7 has the same problem as that of the electron source illustrated in FIG. 6. Specifically, most electrons emitted from the thin diamond films 131 might impinge on the gate electrodes 125.
In the electron source illustrated in FIG. 8, the thin film 141 made of electron emitting material is formed in a cavity at a depth deeper than an interface between the insulating layer 142 and an anode electrode layer 143 to thereby let equipotential surfaces generated in the vicinity of the thin film 141 have a function of focusing emitted electrons.
However, since a step formed at the anode electrode layer 143 has a height small relative to a diameter of the electron emission area, the equipotential surfaces for focusing emitted electrons are bent only in the vicinity of an outer edge of the thin film 141. The equipotential surfaces like this are effective for focusing electrons emitted from an outer edge of the thin film 141, but not effective for focusing electrons emitted from portions of the thin film 141 other than the outer edge thereof. As a result, there is caused a problem that a majority of the emitted electrons impinge on the gate electrode 144, and that electrons having passed through the aperture of the gate electrode 144 are insufficiently focused.